This invention relates to materials having the capacity to form a calcium phosphate (CaP) layer thereon. These materials are ceramic, glass, glass-ceramic or partially crystalline and are bioactive in that they are osteoconductive, osteostimulatory or osteogenic nature when contacted with physiologic solutions, including mammalian bodily fluids and tissues. This invention also provides rapid methods of producing CaP (Hydroxyapatite (HAp) and amorphous HAp) on the surface of a bioactive material.
Certain prior ceramic compositions, especially glass and glass-ceramic, are known to support the bonding, growth or genesis of bone by fostering a supportive environment between the material and living, bone progenitor cells. It is widely recognized that successful bioactive glasses include silica in order to foster the needed supportive environment. These compositions are considered bioactive since they possess surfaces capable of fostering a calcium phosphate layer which, in turn, promotes bone bonding to the material. Bioactive materials of the type described herein include surface-active materials such as those disclosed in U.S. Pat. No. 5,204,106, Schepers, et al., termed 45S5 glass which are of the composition Na2O.CaOP2O5.SiO2.
When implanted, bone formation has been observed in 45S5 as occurring throughout the entire defect through osteogenesis via osteostimulation, assisted by osteoconduction due to the calcium phosphate layer that is formed prior to the central, cellular mediated disintegration. As used herein, osteoconduction generally means the apposition of growing bone to the three dimensional surface of a suitable scaffold provided by a graft. Osteogenesis, as used herein, generally means the process of bone formation through cellular osteoblastic activity. Osteostimulation, as used herein, generally means the promotion of bone growth. Upon central disintegration, the bioactive glass particles are fully transformed into calcium phosphate. The composition and size of the preferred granules in Schepers et al., are such that the particles are gradually transformed as the defect site becomes vascularized and populated with bone tissue-forming cells.
Kokubo et al., xe2x80x9cChemical Reaction of Bioactive Glass and Glass-Ceramics with a Simulated Body Fluid,xe2x80x9d J Mater. Sci.: Mater. Med 3, 79-83 (1992), among others, have described the need for a hydroxyapatite layer to promote bone bonding since hydroxyapatite, whose stochiometric composition is Ca10(PO4)6(OH)2, is the major inorganic component of living bone. It is biocompatible with hard and soft tissues when used as implants. The crystallinity of HAp varies with the maturity of the bone, can contain carbonate, and can be calcium or phosphate deficient.
The drive to form hydroxyapatite on glass materials led Hench and others to use 45S5 glass as a starting material (L. L. Hench, R. J. Splinter, T. K. Greenlee, and W. C. Allen, xe2x80x9cBonding Mechanisms at the Interface of Ceramic Prosthetic Materials,xe2x80x9d J Biomed Mater. Res. Symp. 117-141 (1971)). The theory of apatite formation was extensively explained through the necessary steps of the formation of a hydrated silica gel layer inducing and promoting the nucleation of apatite. When alkali silicate glasses dissolved, alkali ions at the glass surface were selectively exchanged for hydronium ions in the surrounding solution, leaving a hydrated silica gel layer, which served as a nucleation site for CaP crystal deposition, and then subsequent dissolution. Thereafter, most glass compositions used for bioactive purposes contained from 30 to 60 Wt. % Sio2, since it was widely accepted that the formation of hydroxyapatite would occur on particles with a silica-rich layer.
Neo et al., xe2x80x9cDifference in Ceramic-Bone Interface Between Surface-Active Ceramics and Resorbable Ceramics; A Study by Scanning and Transmission Electron Microscopy,xe2x80x9d J. Biomed. Mater. Res., 26, 255-267 (1992), recognize that surface active glasses and glass ceramics bond to bone through an intervening hydroxyapatite layer. When in contact with bodily fluids, the surface of the silicate glass is transformed into a silica-rich layer onto which calcium and phosphorus ions from the surrounding fluids precipitate as a calcium phosphate hydroxyapatite layer. This intervening hydroxyapatite layer is free of collagen and composed of fine granular apatite crystals distinct from those of bone.
The process of synthesizing hydroxyapatite is often time consuming. Processes such as precipitation, hydrolysis, or the use of sol gel usually require several steps over several weeks to form hydroxyapatite, and yet, stochiometric hydroxyapatite cannot be achieved easily. Even 45S5, which provides a surface for the formation of HAp, have in vivo reactions that occur slowly prior to fully bonding to bone. There are few alternative ceramic compositions that are capable of supporting bone growth without silica playing a critical role in the formation process.
Bioactive glasses have been very successful in the promotion of bone growth via a HAp layer, but there are problems. By enlarging the range of starting materials, the drawbacks found in present materials such as those relying chiefly on the presence of silica, may be avoided. Alternative starting materials such as those containing borate may also produce HAp at a faster rate.
Accordingly, it is the object of this invention to provide alternative bioactive materials that are bioactive when used in vivo.
It is also the object of this invention to provide more rapid methods for producing CaP on a bioactive glass.